1. Field of the Invention
This invention relates generally to seismic surveys, and, more particularly, to directional de-signature for seismic signals.
2. Description of the Related Art
Underwater seismic exploration is widely used to locate and/or survey subterranean geological formations for hydrocarbon deposits. A survey typically involves deploying one or more seismic sources and one or more seismic sensors at predetermined locations. For example, a seismic cable including an array of seismic sensors and a seismic source may each be towed along the ocean's surface by a survey vessel. A seismic signal, or shot, provided by the seismic sources generates an acoustic signal that travels to the geological formations, where the acoustic signal is reflected and propagates back to the seismic sensors. The seismic sensors receive the reflected signals, which are then processed to generate seismic data, or traces. Analysis of the traces may indicate probable locations of geological formations and hydrocarbon deposits.
A representation of the acoustic signal known as a signature may also be formed. For example, a so-called far-field signature that is representative of a portion of the acoustic signal that is received by the seismic sensor may be calculated. Historically, an estimate of the far-field signature is removed from the seismic data to reduce interference, a process known as “de-signaturing.” For one example, a model may be used to estimate the far-field signature and de-signature the seismic signal. For a second example, a statistical estimate of the far-field signature may be calculated based upon previous data and the statistical estimate is used to de-signature the seismic signal.
In traditional de-signaturing processes, the far-field signature is estimated by assuming that all the energy in the shot leaves the seismic source and travels vertically downwards. A vertical de-signaturing may then be performed using the estimated far-field signature. However, not all of the far-field signature data that is recorded with the seismic data travels vertically from the seismic source to the receiver. In reality, the energy in the shot may leave the seismic source along a range of takeoff angles and may arrive at the receivers along a variety of emergent angles. Neglecting the takeoff and emergent angles of the signatures may reduce the accuracy of the de-signaturing process. In particular, the phase and high-frequency power of the far-field signature may be incorrectly calculated, which may, in turn, lead to a reduction in the resolution of the seismic survey.